Electroplating method of forming platings of nickel

ABSTRACT

An electroplating method of forming platings of nickel, cobalt, nickel alloys or cobalt alloys with reduced stress in a Watts bath, a chloride bath or a combination thereof, by employing pulse plating with periodic reverse pulses and a sulfonated naphthalene additive. This method makes it possible to deposit nickel, cobalt, nickel alloy or cobalt alloy platings without internal stress.

TECHNICAL FIELD

The present invention relates to an electroplating method of formingplatings of nickel, cobalt, nickel alloys or cobalt alloys in anelectrodepositing bath of the type: Watt's bath, chloride bath or acombination thereof by employing pulse plating with a periodic reversepulse. Current density independence is obtained by means of theinvention, whereby low internal stresses are always rendered, whereverthe measurement thereof is made on a particular member and whichevercurrent density is used.

BACKGROUND ART

The most common electrodepositing baths for nickel electroplating areWatt's baths containing nickel sulfate, nickel chloride and usuallyboric acid; chloride baths containing nickel chloride and boric acid,and sulfamate baths containing nickel sulfamate, nickel chloride andusually boric acid. The latter baths are used for the more complicatedplatings and are difficult and comparatively expensive in use.

Corresponding platings of cobalt may be formed in similar bathscontaining cobalt sulfate and cobalt chloride instead of thecorresponding nickel salts. By adding other metal salts platings ofnickel or cobalt alloys are obtained.

It is known to employ a pulsating current, confer for instance W.Kleinekathofer et al, Metalloberfl. 9 (1982), page 411-420, where pulseplating is used by alternating between equal periods of a direct currentwith a current density of 1 to 20 A/dm² and non-current periods, thepulse frequency being from 100 to 500 Hz. By employing a pulsatingcurrent and as result of the individual current impulses, an increasedformation of crystal nucleuses is obtained, thus rendering a morefine-grained and hard plating.

It is furthermore known to employ pulse plating with periodic reversepulse, i.e. alternating between a cathodic and anodic current. In thecathodic current cycle, the desired plating formation is obtained bymetal deposition, while a portion of the deposited nickel is removed bydissolution in the anodic current cycle, any nodules in the plating thusbeing smoothed. In order to ensure that the, result is a build-up andnot a dissolution of the plating, it is appreciated that the anodic loadis to be less than the cathodic load. This method is e.g. described bySun et al., Metal Finishing, May, 1979, page 33-38, whereby the highestdegree of hardness in the plating is obtained at a ratio between thecathodic and the anodic current density of 1:1 with cathodic cyclesT_(K) of 60 msec. alternating with anodic cycles T_(A) of 20 msec.

U.S. Pat. No. 2,470,775 (Jernstedt et al.) discloses a process forelectroplating nickel, cobalt and alloys thereof in an electrodepositingbath containing chlorides and sulfates of the metals. The plating iseffected by means of reversed pulse resulting in an improved appearance(smoothness and maximum brightness) as well as in an expediteddeposition. An anodic current density is employed of substantially thesame range as the cathodic current density. Various additives arementioned in the U.S. patent, including naphthalene-1,5-disulfonic acid.These additives are referred to as advantageous components, however nodirections are rendered in connection with these additives or elsewherein the patent as to how the mechanical internal stresses are reduced inthe platings resulting from electroplating.

EP patent No. 0.079.642 (Veco Beheer B.V.) relates to pulse plating withnickel in an electrolytic bath of the Watt's bath type comprisingbutynediol or ethylene cyanohydrin as brightener. The deposition ispreferably performed at a pulsating current without anodic cycles, butit is stated that anodic cycles, i.e. reverse pulse, can also beemployed with the same result. It is, however, not possible to use longanodic pulses in a pure Watt's bath without passivating the nickellayer, whereby any further deposition is prevented. Moreover, saidpatent discloses that the frequencies used are in a range from 100 to10,000 Hz.

None of the above mentioned publications relate to internal stresses inplatings. U.S. Pat. No. 3,437,568 relates to a method for measuring theinternal stresses in electroformed parts, but does not advise how toreduce the internal stresses and does not relate to pulse plating,additives or special nickel baths.

DE published specification No. 2.218.967 discloses a bath forelectrodeposition of nickel, to which bath a comparatively large amountof sulfonated naphthalene is added, such as from 0.1 mole/l tosaturation so as to reduce the internal stresses in the platings appliedby electroplating and with a direct current of e.g. 30 or 60 mA/cm²corresponding to 3 to 6 A/dm². According to the publication, theinternal stresses are only reduced from the undesired tensile stressrange to the compressive stress range from 0 to 26,000 psi (approx. 179MPa) by employing this bath.

Usually, the use of said additive only results in a reduction in thestresses in the range from approx. 300 MPa tensile stress to 100 MPacompressive stress and the stress curve is merely moved downward, but isstill a function of the current density, which is a normal condition forany type of nickel bath with or without additives.

The use of the large amount of additive is, however, also encumberedwith several drawbacks, since the additive is expensive, has detrimentaleffects on the environment and may cause damage to the bath.

Thus, an electroplating method, wherein the internal stresses areindependent of the current density, cannot be deduced from the teachingsof DE 2.218.967. When electroplating members of a simple geometricshape, often comparatively modest variations in the current densityoccur over different areas of the surface of the members. However, thisis not possible when dealing with more complicated geometric shapes,wherein the method according to DE 2.218.967 cannot be employed inpractise.

Internal mechanical stress is a problem in all nickel and cobaltdepositions, even though the process can be controlled satisfactorily insome instances (by means of expensive electrolytes (sulfamate bath),temperature control, concentration, etc.) when dealing with simplegeometric shapes. The prior art methods are, however, completelyinapplicable for the manufacture of tools for injection moulding, micromechanical components or similar complicated geometric shapes.

Consequently, it is desirable to provide a method, whereby nickel,cobalt, nickel or cobalt alloys can be deposited with substantiallyreduced or completely without internal stresses--even in complicatedgeometric shapes. It is also desirable that this result is obtainedwhichever current density is used for the deposition.

DISCLOSURE OF THE INVENTION

The present invention relates to an electroplating method of formingplatings of nickel, cobalt, nickel or cobalt alloys in anelectrodepositing bath belonging to the type of a Watt's bath, achloride bath or a combination thereof by employing pulse plating withperiodic reverse pulse, said method being characterised in that theelectrodepositing bath contains an additive selected among sulfonatednaphthalenes.

By employing the method according to the invention internal stresseswhich constitutes a serious problem can be avoided when forming saidplatings on geometric shapes of a more complicated structure.

BEST MODE FOR CARRYING OUT THE INVENTION

Sulfamate baths are more complicated (difficult and more expensive tomaintain), but are generally used to reduce the stress in the platings.However, in a sulfamate bath, it is only possible to obtain platingswith satisfactorily low internal mechanical stresses in case of simplegeometric shapes.

Although sulfamate baths are also used in more complicated geometricshapes, as these present the hitherto best known solution, often theresult is not the optimum due to heavy internal stresses in the platingwhich e.g. may cause deformation or cracks.

Sulfamate baths cannot be used for periodic reverse pulse deposition,sulfur alloyed anodes (2% S) being employed to prevent the sulfamatefrom decomposing into ammonia and sulfuric acid (ruining the bath). Ifthe current is reversed, the cathode coated with non-sulfur alloyednickel or cobalt becomes an anode and the sulfamate is destroyed.

When using a Watt's bath, a chloride bath or a combination thereof, itis not possible to obtain platings using a direct current withouttensile stresses. In sulfamate baths the stress in the plating--fromcompressive stress through stress-free to tensile stresses--depends onthe cathodic current intensity I_(K). Consequently, on simple geometricshapes stress-free platings can be obtained by means of a sulfamate bathat a specific I_(K) which depends on the temperature and may e.g. be ofapproximately 10 A/dm², but on more complicated geometric shapes thiscurrent intensity I_(K) is not distributed evenly on the entire surfaceof the member and causes internal stresses.

The use of the combination according to the invention has surprisinglyshown that the internal stresses are very small and independent of thecathodic current intensity I_(K) and thus of the current distribution onthe surface. As a result, low internal stresses are obtained wherever onthe member the internal stress is measured and independent of the actuallocal current densities.

In this manner, the invention renders it possible to manufacturecomplicated geometric shapes completely without or with considerablyreduced internal stresses in the plating.

As additive in the method according to the invention, sulfonatednaphthalene is used, i.e. naphthalene sulfonated with from 1 to 8sulfonic acid groups (--SO₃ H), preferably with 2 to 5 sulfonic acidgroups, most preferred 2-4 sulfonic acid groups. In practice, asulfonated naphthalene product usually comprises a mixture of sulfonatednaphthalenes with various degrees of sulfonation, i.e. the number ofsulfonic acid groups per naphthalene residue. Moreover, several isomericcompounds may be present for each degree of sulfonation.

Typically, the used sulfonated naphthalene sulfonide has a degree ofsulfonation on average corresponding to from 2 to 4.5 sulfonic acidgroups per molecule, e.g. 2.5- to 3.5 sulfonic acid groups per molecule.

In the presently preferred embodiment of the invention, a mixture ofsulfonated naphthalenes is used as sulfonated naphthalene additive, saidmixture according to analysis containing approximately 90% ofnaphthalene trisulfonic acid, preferably comprisingnaphthalene-1,3,6-trisulfonic acid and naphthalene-1,3,7-trisulfonicacid.

The naphthalene residue in the sulfonated naphthalene additive isusually free of other substituents than sulfonic acid groups. Any othersubstituents may, however, be present provided that they are notdetrimental to the beneficial effect of the sulfonated naphthaleneadditive on minimizing the internal stresses in the plating formed byemploying pulse plating.

In a particular preferred embodiment according to the invention, thesulfonated naphthalene additive is used in the electroplating bath inthe amount of 0.1 to 10 g/l, more preferred in an amount of 0.2 to 7.0g/l and most preferred in an amount of 1.0 to 4.0 g/l, e.g. around 3.1g/l.

Moreover, according to the invention the bath composition preferablycontains 10-500 g/l of NiCl₂, 0-500 g/l of NiSO₄ and 10-100 g/l of H₃BO₃, more preferable 100-400 g/l of NiCl₂, 0-300 g/l of NiSO₄ and 30-50g/l of H₃ BO₃ and preferable 200-350 g/l of NiCl₂, 25-175 g/l of NiSO₄and 35-45 g/l of H₃ BO₃, for instance about 300 g/l of NiCl₂, 50 g/l ofNiSO₄ and 40 g/l of H₃ BO₃.

It has proved advantageous that the anodic current density I_(A) is atleast 1.5 times the cathodic current density I_(K), more preferable whenI_(A) ranges from 1.5 to 5.0 times the I_(K) and most preferable whenI_(A) is 2 to 3 times the I_(K).

In a preferred embodiment, the method according to the invention may becharacterised in that the pulsating current is made up of cathodiccycles, each of a duration T_(K) of from 2.5 to 2000 msec. and at acathodic current density I_(K) of 0.1 to 16 A/dm² alternating withanodic cycles, each of a duration of from 0.5 to 80 msec. and at ananodic current density I_(A) of 0.15 to 80 A/dm². A more preferableembodiment according to the invention is obtained when among the pulseparameters the I_(K) ranges from 2 to 8 A/dm², the T_(K) ranges from 30to 200 msec., the I_(A) ranges from 4 to 24 A/dm² and T_(A) ranges from10 to 40 msec. A particular preferred embodiment is obtained when I_(K)is from 3 to 6 A/dm², T_(K) is from 50 to 150 msec., I_(A) is from 7 to17 A/dm² and T_(A) is from 15 to 30 msec., e.g. when I_(K) is 4 A/dm²,T_(K) is 100 msec., I_(A) is 10 A/dm² and T_(A) is 20 msec.

EXAMPLES Example 1

A nickel bath containing 300 g/l of NiCl₂.6H₂ O and 50 g/l of NiSO₄.6H₂O was admixed, and to which bath 40 g/l of H₃ BO₃ and 3.1 g/l ofsulfonated naphthalene additive of technical grade comprising 90%naphthalene-1,3,6/7-trisulfonic acid were added.

Nickel was deposited on a steel strip fixed in a dilatometer so that theinternal stresses in the deposited nickel can be measured as acontraction or a dilation of the steel strip. The temperature of thebath was 50° C. When nickel was deposited from said bath at a pulsatingcurrent having the cathodic pulse of 100 msec. and 3.5 A/dm² followed byan anodic pulse of 20 msec. and 8.75 A/dm², the internal stresses weremeasured to be 0 MPa or less than the degree of accuracy of theapparatus of approximately ±10 MPa.

Example 2

Following the method according to Example 1 with the exception that only1.1 g/l of the same sulfonated naphthalene additive was used, the sameresult was obtained as in Example 1, i.e. that the internal stresseswere to measure to 0 MPa or less than the degree of accuracy of theapparatus of approximately ±10 MPa.

Example 3

Following the method according to Example 2 with the exception that theanodic current density I_(A) and the cathodic current density I_(K) wasset at 1.25 A/dm² and 0.5 A/dm² respectively, the same result as inExample 1 was obtained, i.e. that the internal stresses were measured to0 MPa or less than the degree of accuracy of the apparatus ofapproximately ±10 MPa.

Example 4

Following the method according to Example 3 with the exception that theanodic current density I_(A) and the cathodic current density I_(K) wasset at 18.75 A/dm² and 7.5 A/dm² respectively, the same result as inExample 1 was obtained, i.e. that the internal stresses were measured to0 MPa or less than the degree of accuracy of the apparatus ofapproximately ±10 MPa.

Example 5

Using the method according to Example 1, in which the nickel bathcontaining 300 g/l of NiCl₂.6H2O and 50 g/l of NiSO₄.6H₂ O issubstituted by 300 g/l of CoCl₂.6H₂ O and 50 g/l of CoSO₄.6H₂ O and thesame amount of H₃ BO₃ and sulfonated naphthalene additive, similarcobalt platings can be produced which are expected to have the similarlow internal stresses.

Example 6

Following the method according to Example 5 with the exception that 1.1g/l of sulfonated naphthalene additive was used, similar stress-freecobalt platings may be expected.

Example 7

Following the method according to Example 6 with the exception that theanodic current density I_(A) and the cathodic current density I_(K) wasset at 1.25 A/dm² and 0.5 A/dm² respectively, similar stress-free cobaltplatings can be expected.

Example 8

Following the method according to Example 7 with the exception that theanodic current density I_(A) and the cathodic current density I_(K) wasset at 18.75 A/dm² and 7.5 A/dm² respectively, similar stress-freecobalt platings are expected.

Comparison Examples Comparison Example 1

Employing the same set-up and materials as in Example 1, but at a directcurrent of 4 A/dm², the internal stresses for comparison with saidExample were measured to 377 MPa.

Comparison Example 2

Employing the same set-up and materials as in Example 2, but using adirect current of 7.5 A/dm², the internal stresses were measured to 490MPa.

Comparison Example 3

Employing the same set-up and materials as in Example 2, but insteadusing a pulsating current without reverse pulse (I_(K) =3.5 A/dm², T_(K)=100 msec., I_(A) =0 A/dm², T_(A) =20 msec.), the internal stresses weremeasured to 410 MPa.

We claim:
 1. An electroplating method comprising forming platings ofnickel in a chloride-sulfate-boric acid electrodepositing bath byemploying pulse plating with periodic reverse pulsating current made upof cathodic cycles, each of a duration T_(K) of from 2.5 to 2000 msec.at a pulsating or uniform cathodic current density I_(K) of 0.1-16 A/dm²alternating with anodic cycles, each of a duration T_(A) of from 0.5 to80 msec. at an anodic current density I_(A) of 0.15-80 A/dm², whereinthe electrodepositing bath contains sulfonated naphthalene as anadditive in an amount of 0.1 to 10 g/l and the anodic current densityI_(A) at least 1.5 times the cathodic current density I_(K).
 2. Methodaccording to claim 1, wherein the sulfonated naphthalene has an averagedegree of sulfonation of 1 to 6 sulfonic acid groups per naphthaleneresidue.
 3. Method according to claim 2, wherein the sulfonatednaphthalene has an average degree of sulfonation of 2 to 5 sulfonic acidgroups per naphthalene residue.
 4. Method according to claim 2, whereinthe sulfonated naphthalene has an average degree of sulfonation of 2 to4.5 sulfonic acid groups per naphthalene residue.
 5. Method according toclaim 2, wherein the sulfonated naphthalene has an average degree ofsulfonation of 2.5 to 3.5 sulfonic acid groups per naphthalene residue.6. Method according to claim 2, wherein the sulfonated naphthalenecomprises about 90% of naphthalene trisulfonic acid, wherein saidnaphthalene trisulfonic acid is a mixture ofnaphthalene-1,3,6-trisulfonic acid and naphthalene-1,3,7-trisulfonicacid.
 7. Method according to claim 1 wherein the bath compositioncomprises 10 to 500 g/l of NiCl₂, 25 to 500 g/l of NiSO₄ and 10 to 100g/l of H₃ BO₃.
 8. Method according to claim 1, wherein the anodiccurrent density I_(A) is from 1.5 to 5.0 times the cathodic currentdensity I_(K).
 9. Method according to claim 1, where the pulsatingcurrent is made up of cathodic cycles, each of a duration T_(K) of from30 to 200 msec. at a cathodic current density I_(K) of 2-8 A/dm²alternating with anodic cycles, each of a duration T_(A) of from 10 to40 msec. at an anodic current density I_(A) of 5 to 20 A/dm².
 10. Methodaccording to claim 9, wherein the pulse parameters I_(K), T_(K), I_(A),T_(A) are 4 A/dm², 100 msec., 10 A/dm² and 20 msec., respectively. 11.Method according to claim 1, wherein the bath composition comprises 100to 400 g/l of NiCl₂, 25 to 300 g/l of NiSO₄ and 30-50 g/l of H₃ BO₃. 12.Method according to claim 1, wherein the bath composition comprises 200to 350 g/l of NiCl₂, 25 to 175 g/l of NiSO₄ and 35 to 45 g/l of H₃ BO₃.13. Method according to claim 1, wherein the anodic current densityI_(A) is from 2.0 to 3.0 times the cathodic current density I_(K). 14.Method according to claim 1, wherein the additive is used in an amountof 0.2 to 7.0 g/l.
 15. Method according to claim 1, wherein the additiveis used in an amount of 1 to